This invention generally relates to poppet valves, and is specifically concerned with a stepper motor controlled valve having a resilient linkage assembly for depressing the valve element into the valve seat with a constant force regardless of variations in the distance between the valve element and valve seat caused, for example, by thermal expansion or contraction of the valve body. The poppet valve of the invention advantageously provides not only positive shut-off, but throttling and soft-start capabilities as well.
Control valves for the precision regulation of gas flows and maintenance of vacuum and near-vacuum conditions are well known in the prior art. Such control valves are used, for example, in devices for manufacturing semiconductor components and integrated circuits on silicon chips. In such applications, three different types of control valves are frequently necessary, including a shut-off valve for positively isolating the vacuum system for the ambient atmosphere, a throttling valve for modulating a flow of gas from one part of the system to another, and a soft-start bypass valve for facilitating slow pump-downs from atmospheric pressures to minimize turbulence and particle disturbance within the vacuum system. The ability to positively isolate the vacuum system from the ambient atmosphere is a necessary precondition if vacuum or low pressure integrity is to be maintained. The ability to precision modulate a flow of gas in such systems is particularly important in the implementation of manufacturing processes such as chemical vapor deposition, sputtering, and ECR deposition. The ability to implement slow pump-downs in such systems is also important, as a turbulent flow of gas through the valve can cause the dislodgement of particulate matter in the system onto the silicon chips, thereby spoiling the final product.
Thus far, no one single valve structure has been discovered which can satisfactorily perform all three functions of positive isolation, gas throttling, and soft-start capabilities. While prior art poppet-type valves are capable of providing excellent isolation between the vacuum system and the ambient atmosphere, they are not capable of providing the type of precision throttling and soft-start capabilities required in such systems, largely due to the fact that once the valve element is withdrawn a short distance from the valve seat, such valves instantly become highly conductive to gas flow. While it is conceivable that improved throttling characteristics might be obtained with improved distance control between the valve element and the valve seat, the required degree of the precision control of such small distances has in the past been rendered near impossible due to the dimensional changes of the valve body and valve stem due to thermal expansion or contraction as heated gases flow through the bodies of such valves. Specifically, the applicant has observed that when the body of the valve is heated (as may be done to prevent the condensation of silicon nitride out of a gas during a flow forming process) such heat typically does not raise the temperature of the valve stem at the same rate as the valve body due to the isolating bellows which typically circumscribes and contains the valve stem in such valves. The resulting thermal differential expansion of the valve body relative to the valve stem can not only alter the flow characteristics of such valves; it can (under certain conditions) actually jeopardize the integrity of the positive seal desired when such valves are used solely for isolation purposes.
While other types of prior art control valves are known which are capable of providing adequate throttling characteristics, such valve designs (which may include, for example, butterfly-type valves, vane valves, gate valves or angle valves) typically employ wiping seals whose outer surfaces continually wear down with use. The wearing down of these seals continually changes the flow characteristics of these valves, which must be compensated for by either the use of relatively expensive control mechanisms or the constant adjustment of a simpler control mechanism by the operator of the valve. In many cases, wiping the seal against contamination renders the seal useless as a vacuum seal.
Clearly, there is a need for a single type of valve which is capable of performing all three functions of positive shutoff, throttling, and soft-start in order to obviate the need for three separate kinds of valves in such vacuum systems. Ideally, such a valve should be simple in construction and controllable by means of relatively simple and inexpensive control circuits. Finally, it would be desirable if such a valve did not employ wiping seals so that all of the problems associated with the wearing down of such seals over time would be avoided or at least minimized.